Viscoelastic parameters of tissue include the elasticity and viscosity of the tissue. These viscoelastic parameters can vary between healthy and diseased regions of tissue. For example, cancerous breast tumors are approximately eight times stiffer than benign lesions while benign lesions are approximately four times stiffer than normal breast tissue (J. Ophir, I. Cespedes, B. Garra, H. Ponnekanti, Y. Huang and N. Maklad, “Elastography: ultrasonic imaging of tissue strain and elastic modulus in vivo,” European Journal of Ultrasound, vol. 3, no. 1, pp. 49-70, January 1996). Therefore, a doctor examining a patient for a tumor will typically press down on various regions of the patient's body, searching for relatively stiff (highly elastic) regions that could signify the pathology.
Viscosity is also an important mechanical property when assessing tissue health. For example, cancerous lesions with a high blood vessel concentration have a higher viscosity than surrounding, healthy tissue (R. Sinkus, M. Tanter, T. Xydeas, S. Catheline, J. Bercoff, and M. Fink, “Viscoelastic shear properties of in vivo breast lesions measured by MR elastography,” Magnetic Resonance Imaging, vol. 23, no. 2, pp. 159-165, 2005).
Tissue can be modeled as being composed of a series of interconnected elements, with each element being composed of a mass (m) coupled to a parallel arrangement of a spring having a spring coefficient (k) and a damper having a damping coefficient (b). The spring is used to model the elasticity of the tissue, which is reflected in the tissue's resilience to an applied external force. The damper is used to model the viscosity of the tissue, which is reflected in the tissue's internal friction and relaxation effects. The mass is used to model the density of the tissue. When relatively small applied forces are applied and relatively small displacements result, the force-velocity relationship of the dampers is linear, as is the force-displacement relationship of the springs. All objects with mass exhibit a linear relationship between force and acceleration. One, two or three dimensional arrays of elements are also possible. For an example of a two dimensional array, see FIG. 10 of U.S. Pat. No. 7,731,661 to Salcudean et al., the entirety of which is herein incorporated by reference.
To estimate relative values for elasticity and viscosity in tissue, vibrations can be applied to the tissue and the resulting tissue displacement observed. This can be done over a one, two or three dimensional region and using a variety of imaging modalities; ultrasound and magnetic resonance imaging (MRI) are both modalities that have been used. See, for example, U.S. Ser. No. 10/963,795 to Salcudean et al., and Emre Turgay, Septimu Salcudean, and Robert Rohling, “Identifying the mechanical properties of tissue by ultrasound strain imaging,” Ultrasound in Med. & Biol., vol. 32, no. 2, pp. 221-235, 2006, the entirety of which is herein incorporated by reference. By “relative value”, it is meant a value for elasticity or viscosity at a certain location within the tissue that is expressed relative to another location within the tissue. A “relative value” is in contrast to an “absolute value”, which is a value for elasticity or viscosity that is expressed independently from any other location within the tissue. For example, an absolute value for viscosity is 10 Pas, whereas a relative value for viscosity is 10·x Pas, where “x” is a baseline value for viscosity measured elsewhere in the tissue.
However, prior art methods are often slow and cannot image a tissue region in real-time, can require expensive machinery, or can only determine relative values of viscoelastic parameters.
Consequently, there exists a need for a method and system that addresses one or more deficiencies in the prior art.